The three dots you are referring to are called ellipsis. In mathematics, an ellipsis is used to indicate that a set continues indefinitely or that a pattern carries on. For example, if we write the set of natural numbers as {1, 2, 3, …}, the ellipsis signifies that the sequence continues indefinitely with 4, 5, 6, and so on.
Using ellipsis is a concise way to represent infinite sequences or patterns without requiring us to list every single element, making it easier to communicate certain mathematical ideas clearly.